1. Field of the Invention
This invention relates to microstructure optical fibers and, more particularly, to the suppression of preselected transverse modes in such fibers.
2. Discussion of the Related Art
Microstructure optical fibers (MOFs) guide light in a core region (e.g., an aircore) surrounded by a periodic index cladding region. Illustratively, the cladding region includes a matrix (or lattice) that defines a periodic array of air holes. When properly designed, an MOF can be made to exhibit bandgap confinement; that is, a range of wavelengths that support and guide the propagation of light modes bounded by ranges of wavelengths that do not support such propagation. Such fibers, are often referred to as bandgap fibers, typically have a core region that has a lower refractive index than that of the cladding region. In contrast, MOFs can also be designed to exhibit more traditional index guiding, in which case the core region has a higher refractive index than that of the cladding region.
Progress in aircore and other bandgap fibers has demonstrated that they can combine many of the material and manufacturing advantages of the standard fiber draw process with unique capabilities not possible in standard fibers. Aircore fibers have particularly interesting potential in data transmission, since the nonlinearity of aircore fibers is much smaller than standard fibers, dramatically changing the cost and performance of a transmission link. However, any long-distance transmission fiber must compete against the extremely low loss of standard fibers. Despite rapid improvements, it has been suggested that aircore fiber losses are approaching a fundamental limit and can be improved further only by increasing the core size or finding improved materials. [See, P. J. Roberts, et al., “Ultimate low loss of hollow-core photonic crystal fibres,” Opt. Express, Vol. 13, p. 236 (2005), which is incorporated herein by reference.] Consequently, it would be very desirable to utilize larger core sizes, but larger cores generally support higher-order transverse modes (HOMs) and therefore experience well-known problems attendant such modes.
Hereinafter, the term mode shall mean transverse mode in both the singular and the plural.
Therefore, a need remains in the art for a bandgap MOF that has a relatively large core size yet suppresses HOMs.